With the proliferation of wireless communications technology, more and more devices are equipped with wireless communications capabilities, such as Bluetooth, IEEE 802.11, or Zigbee. With wireless communications capabilities, the devices are no longer tethered to a fixed location, and can freely move about together with the users. This gives rise to an important application for wireless devices: to be able to provide location and navigation related services. The ubiquity of Global Positioning System (GPS) has allowed users to tell their current locations through the use of GPS-equipped devices. However, in urban areas, the presence of high-rise buildings blocking line-of-sights to GPS satellites has resulted in difficulties of locating one's position using GPS. Some cellular devices use cellular base stations to assist in GPS positioning. In recent years, the high availability of wireless local area network (WLAN) hotspots in urban cities has led to the use of WLAN to locate devices in outdoor environment as well. For example, the Patent Document 1 (US Patent Application Publication US2007/0004428A1) teaches the method of monitoring WLAN access points in a target area to indicate movements.
While the afore-mentioned methods are shown to be successful in outdoor environment, there has been increasing interest to perform location determination in indoor situation where these methods are either impractical or have poor performances. To extend these method, one approach taught by the Patent Document 2 (U.S. Pat. No. 7,412,248B2) is to estimate the virtual location of base stations by a mobile device while the mobile device is able to tell its own location through, say, GPS, and then to make use of these virtual locations to continue to determine the mobile device's location when GPS is no longer available. The Patent Document 3 (U.S. Pat. No. 7,570,927B2) differentiates between location-capable nodes (i.e. nodes that can determine their own locations independently, e.g. using GPS) and location-incapable nodes. It then discloses a method of determining the location of location-incapable nodes through transmission of location determination information by the location-capable nodes. These methods require there to be nodes with location determination capabilities that are independent from the wireless communications, which may not always be possible in some deployment scenarios. It would be desirable and have wider applicability if the location of the mobile device can be determined only from parameters related to the wireless transmission.
One possible approach to determine location only from parameters of the wireless communications is to make use of the Received Signal Strength Indication (RSSI), Angle of Arrival (AOA), Time of Arrival (TOA), or Time Differential of Arrival (TDOA) to infer the distance between a mobile device and a fixed anchor point. Only methods that use only RSSI can have wide applicability, as specialized or expensive hardware are required to perform the measurements of the other wireless transmission parameters, which cannot be taken using readily available wireless chipset on the market.
One popular technique utilizing only the RSSI is to use what is known as a fingerprinting method, which involves observing and recording the RSSI from a set of base stations at different points of the area where location detection is needed. This set of recorded RSSI forms the fingerprint of that particular point, hence the name of this method. Thereafter, when the mobile device needs to know its position, it compares its currently observed RSSI from the same set of base stations with the recorded fingerprint, and determines its location as the point where the observed RSSI least deviates from the recorded fingerprint. One variation of this fingerprinting method is disclosed in the Patent Document 4 (U.S. Pat. No. 7,660,591B1). In the Patent Document 4 (U.S. Pat. No. 7,660,591B1), in addition to taking the measurements of RSSI at a plurality of locations, the known transmit power is also subtracted from each of the RSSI measurements to produce a Propagation Loss Vector (PLV) for each location of the plurality of locations. This basically removes variations due to the transmitter's transmit power when using the fingerprinting method.
The main problem with the fingerprinting method is that prior RSSI needs to be observed and recorded at every point in the area of location determination, making it not feasible in some deployments. Some other techniques tries to reduce or eliminate the burden of having to perform prior calibration is to make use of probabilistic methods in addition to comparing between observed RSSI and recorded fingerprints. For example, the Patent Document 5 (U.S. Pat. No. 7,403,784B2) discloses a method for estimating a location of a plurality of wireless terminals where a Bayesian algorithm is applied to the signal strength measurements to estimate the location of each wireless terminal. Further, the Patent Document 6 (PCT Patent Application Publication WO 2007/021071A1) teaches a method of determining a location of a mobile communication terminal where weights are assigned to the received signal strength from different base stations. The weights are determined statistically. In the Patent Document 7 (U.S. Pat. No. 7,821,453B2), an iterative method is disclosed where Kalman filter is used to continuously refine the estimated position of a plurality of nodes separately such that in each iteration, the inter-node distance between the said plurality of nodes is used in the estimation of the position of one of the nodes.
Other techniques include combination of different location determination approaches to achieve better accuracy as described in the Patent Document 8 (U.S. Pat. No. 7,751,827B2), using different wireless communications technologies as disclosed in the Patent Document 9 (US Patent Application Publication US2010/0105409A1).